It seems that VLC doesn't like the M3U format that VDR uses for recordings.
Looking at a sample recording entry:
#EXTM3U
#EXTINF:1800,The Repair Shop
#EXT-X-TARGETDURATION:1800
#EXT-X-STREAM-INF:PROGRAM-ID=da01a7de69dfd8d09241c045d6fbaeb6,BANDWIDTH=5115
#EXT-X-PROGRAM-DATE-TIME:2018-03-23T18:30:00+0000
http://192.168.0.1:9981/dvrfile/da01a7de69dfd8d09241c045d6fbaeb6?ticket=0b9bdcdec9f7f95392bf82f34d9908a6e99697af
It is necessary to delete the lines beginning "EXT-X-", VLC will then recognise the file as a playlist.
The recordings list from TVHeadend also includes recordings to be made in the future (ie timers). These have "bandwidth=0" in the EXT-X-STREAM line and should also be deleted.